A) Field of the Invention
The present invention relates to a photoelectric conversion device, and more particularly to a semiconductor photoelectric conversion device suitable for miniaturization.
B) Description of the Related Art
Most of semiconductor photoelectric conversion devices use a pn junction diode as a photoelectric conversion element and read accumulated electrons to generate a signal representative of the amount of received light. When light becomes incident upon a pn junction diode, photoelectric conversion occurs to generate pairs of electron-hole and electrons are accumulated in an n-type region.
In a CCD semiconductor photoelectric conversion device, accumulated electrons are transferred by charge transfer channel CCDs and amplified by an output amplifier to acquire image signals. In a MOS semiconductor photoelectric conversion device, accumulated charges are amplified by a MOS transistor and picked up via a wiring line. In both the cases, functional devices such as CCDs and transistors are disposed near at a pn junction diode or photoelectric conversion element.
These functional devices include pn junction diodes which generate charges corresponding to the amount of incident light. It is desirable that these charges are eliminated because they add noises to the light reception signal obtained from the pn junction diode. In order to eliminate the charges, a light shielding film is disposed above a semiconductor substrate. The light shielding film has a light transmission window above each photoelectric conversion element and shields light which otherwise enters the peripheral functional devices.
A micro lens is disposed above the light shielding film in order to make light passed through a taking lens efficiently enter each photoelectric conversion element. Incident light passed through the micro lens is converged and enters the widow of the light shielding film. If the micro lens does not exist, light is guided to the window in order not to allow the light to enter the light shielding film and become invalid incidence light to thereby improve photoelectric conversion efficiency.
An example of prior art will be described with reference to the accompanying drawings.
FIG. 10A is a schematic cross sectional view showing the structure of a CCD semiconductor photoelectric conversion device according to the prior art. A silicon substrate 10 has a p-type layer 2 on an n-type region 1. The p-type layer 2 has an n-type region 3 constituting a photodiode and an n-type region 5 constituting a vertical charge transfer channel VCCD. On the n-type region 3, a p+-type region 4 is formed to bury the photodiode. A p-type region 6 is formed on the bottom surface of the n-type region 5 to electrically separate the vertical charge transfer channel, and a p+-type region 7 functioning as a channel stopper is formed between adjacent columns.
On the surface of a silicon substrate 10, a silicon oxide film 11 thermally oxidized is formed. On the silicon oxide film 11, a charge transfer electrode 12 of a so-called double polysilicon structure is formed to drive the vertical charge transfer channel. The upper space of the photodiode is made open in order to introduce light. After the surface of the charge transfer electrode 12 is covered with a silicon oxide layer 14, a light shielding film 15 made of tungsten or the like is formed, the light shielding film having a window above each photodiode.
An insulating layer 17 having a flat surface is formed covering the light shielding film 15, the insulating layer being made of a boron—phosphorous—silicon oxide (borophosphosilicate glass, BPSG) or the like. Color filters 31 are formed on the insulating layer 17. The color filter layer 31 is covered with a surface planarizing layer 32 such as resist, and thereafter micro lenses 33 are formed on the surface of the surface planarizing layer 32 by using resist material or the like.
With these processes, a semiconductor photoelectric conversion device with color filters is formed. Color filters are omitted for a three-plate type photoelectric conversion device. Incidence light upon the photodiode is limited by the light shielding film, and only the light passed through the window of the light shielding film can enter the photodiode.
FIG. 10B is a plan view showing an example of the layout of a semiconductor image pickup device. A number of photodiodes PD are disposed in rows and columns in a tetragonal matrix shape. A vertical charge transfer channel VCCD is formed adjacent to each photodiode column. A horizontal charge transfer channel HCCD is coupled to one ends of the vertical charge transfer channels VCCDs.
FIG. 10C is a plan view showing another example of the layout of a semiconductor image pickup device. A number of photodiodes PD are disposed shifted by a half pitch in the row and column directions, and has a so-called honeycomb layout. VCCD extends in a zigzag way between pixels of the honeycomb structure.
FIG. 11 is a schematic cross sectional view showing the structure of another CCD semiconductor photoelectric conversion device according to the prior art. As compared to the structure shown in FIG. 10A, a low refractive index insulating layer 18 and a high refractive index insulating layer 19 are inserted between the light shielding film 15 and insulating layer 17. The high refractive index insulating layer 19 has a downward convex shape to provide a lens function.
FIGS. 12A, 12B, 12C and 12D show the results of optical path calculations of the structures shown in FIGS. 10A and 11. FIGS. 12A and 12B show the calculation results of the structure shown in FIG. 10A when incidence light enters at an incidence angle of 0 and 10 degrees. FIGS. 12C and 12D show the calculation results of the structure shown in FIG. 11 when incidence light enters at an incidence angle of 0 and 10 degrees. In the structure shown in FIG. 10A, as the incidence angle becomes oblique, the incidence light enters not only the window of the light shielding film but also other regions so that the efficiency lowers.
If each pixel of a photoelectric conversion device is made small by proportionally reducing the size of constituent elements, it is expected that the sensitivity proportional to a pixel size is obtained without lowering the sensitivity per unit area. However, there is the phenomenon that as the opening size of a light shielding film is reduced, the sensitivity lowers at a higher rate than the reduction rate of the pixel size. This phenomenon that the sensitivity lowers at a higher rate than the reduction rate of a light reception area becomes conspicuous as the incidence angle becomes large.